Energy Of A Single Photon In Em Radiation?

  1. Energy Of A "Single" Photon In Em Radiation?

    Is the energy of all photons in em radiation same? That is, say light differs from radio waves only in the number of photons per second
  2. jcsd
  3. analogdesign

    analogdesign 834
    Science Advisor

    No, the energy of a photon is proportional to its frequency. That is one reason why gamma radiation is dangerous but radio waves are harmless.
  4. How can a single photon have frequency? Imagining a single photon travelling sinusoidally(?) it does not make sense to think that its energy has got anything to do with the sine wave. How can its energy vary just depending on its path?
  5. Nugatory

    Staff: Mentor

    It may not make sense, but that's still the way the world works. The most convincing evidence of this was discovered around the end of the 19th century (google for "photoelectric effect einstein").

    EDIT: The photon most certainly does not travel a sinusoidal path. But it still has a frequency and that frequency is proportional to its energy.
    Last edited: Aug 29, 2013
  6. jtbell

    Staff: Mentor

    No, photons do not travel in sinusoidal paths. Neither do electrons or other particles, which also have a "wave function" associated with them. There is no generally accepted answer to what this wave function "really really is," with the result that half the threads in our Quantum Physics forum are related to this puzzle. All we know for sure is that we can define this wave function, and do certain mathematical operations on it to predict the results of experiments very successfully, in a statistical sense at least.
  7. 1.Imagining some kind of wavy pattern are there 2 streams of photons moving perpendicular to each other?or is it merely after hitting an electron that it induces E and B?

    2.Why would photons move in wavy pattern at all,cant they move in a straight line?
  8. ZapperZ

    ZapperZ 30,743
    Staff Emeritus
    Science Advisor
    Education Advisor

    Did you even read the post right before this by jtbell? Only you think that photons move in a "wavy pattern".

  9. f95toli

    f95toli 2,473
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    Gold Member

    Photons do not move in a "wavy pattern". That said, we can't really say anything about HOW they move. Photons are fundamentally quantum mechanical "objects" and it is not possible build up any inutition about how they "really" move, or even what they are (the same can be said about just about everything else as well).

    We have extremelly good mathematical models, so the problem is NOT that we do not understand what photons are. But what these models describe is so different from our everyday macroscopic world that it is extemely difficult to get any inuitive grasp of them
  10. I was having that picture of perpendicular electric and magnetic fields in mind...what do they actually mean?

    Cant we make a common sensical assumption that they( or the wavefunction that represents it) move in a straight line.
  11. I am sorry if had hurt your sentiments regarding photon...:wink::smile:
  12. No. There are many instances where that can be experimentally demonstrated to be false. Even a single slit experiment will get photons travelling in bent lines, and two slit or gradient experiments can show that a single photon travels along multiple paths.
    1 person likes this.
  13. ZapperZ

    ZapperZ 30,743
    Staff Emeritus
    Science Advisor
    Education Advisor

    (Scratches head)

    Er..... Alright then!

    But you still didn't answer the question. It is hard to know if you just didn't read the responses you've been given, or you didn't understand what you read. It has nothing to do with "hurt sentiments", whatever those are.

  14. Yes I have read it.I made the mistake of thinking of frequency as only the number of particles passing through a point for a while.
    Last edited: Aug 30, 2013
  15. What does that common representation of em wave as perpendicular electric and magnetic field imply?

    Is there any particular order of emission that is radio waves, microwaves etc in that order or are they emitted randomly?
  16. jtbell

    Staff: Mentor

    It means that if you "shine" an electromagnetic wave on (say) an electron, it exerts electric and magnetic forces on the electron.

    What do you mean by "order of emission?"
    Last edited: Aug 30, 2013
  17. Classically an EM wave is a propagating EM field in which the electric and magnetic fields are mutually perpendicular and perpendicular to the direction of propagation. That is the classical picture, but you really shouldn't mix the classical and quantum pictures.

    I don't know what you are asking here. Radio waves are low frequency, microwaves are slightly higher, infrared is higher than that, then visible light, ultraviolet, x-rays, and gamma-rays. Their emission is not random, but the way you phrased the question is confusing.
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  18. So there is only one stream of photons and no perpendicular streams ?
    What I meant by order of emission is, is there some process sequentially producing photons of different frequency according to em spectrum? or are photons of different frequencies being produced at random?
  19. You could do a frequency sweep, but this would typically be done with RF where the quantum mechanical description (photons) is not terribly helpful and you are generally better sticking with a classical description.

    My personal recommendation is that it seems like you should learn classical EM before attempting to learn quantum electrodynamics.

    It might help if you identified the system you are interested in. The mechanism for producing photons is different if you are talking about a radio antenna, an incandesent light, an x-ray tube, or a flourescent substance.
  20. jfizzix

    jfizzix 468
    Science Advisor
    Gold Member

    A photon is a hard thing to pin down (I'm a grad student in quantum optics, so it's something I think about a lot).

    A photon follows most of the same quantum rules that other quantum particles do, and in that sense, it's just as hard to talk about a single photon as it is a single electron, or any other particle.

    We can only infer properties about photons through the experiments we perform (like everything else). Photons are especially hard to discuss because most of the time, by measuring a photon, you destroy it (i.e. it gets absorbed by a detector).

    The full-blown mathematical treatment of a photon (in my field) describes it as a particular kind of quantum state of the electromagnetic field. It's a very low energy state with only one photon in the field, but it is more than the vacuum. You can have 2-photon states, 3-photon states, coherent states, thermal states, entangled states, and many more yet to be thought of.

    You can of course make approximations, which is why we don't need quantum field theory to design mirrors and lenses and such (yet). Almost all of the time (when you see quadrillions of photons) Classical Electromagnetism will describe everything just fine (easier said than done).
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  21. jfizzix

    jfizzix 468
    Science Advisor
    Gold Member

    The photoelectric effect is a simple experiment explaining how photons can have an energy proportional to their frequency, provided one is okay with light being divided up into chunks known as photons.

    That being the case, scientists have shown that with high frequency light, you can knock electrons off pieces of metal. The brighter that light (the more photons), the more electrons get knocked off, and we can measure these electrons hitting detectors.

    However, at low frequencies, no electrons get knocked off the metal, no matter how bright the light is.

    We know that if the photon has enough energy, it can knock an electron off, and it can't if it doesn't.

    More importantly, we can measure the kinetic energy of the electrons that get knocked off and see that the higher the frequency of the light, the higher the kinetic energy of the ejected electrons.

    This relationship between electron energy and frequency of the incident light is precisely what we expect if we say the energy of a photon is proportional to the frequency of the light it's a part of.

    It's hard to think of photons on their own, but if a photon is just a very tiny part of a beam of light, then it makes sense to speak of the frequency of a photon as the frequency of the light it is part of.
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